When defining caries increments in children and young adults it is customary to refer to DMFS increments as net increment, NI(DMFS), and it is defined as the number of sound surfaces which become "decayed" (SD+SF+SM) minus the number of "decayed" surfaces that become sound (DS+FS+MS). If increment is defined as the difference in time-specific DMFS scores, ie. DIF(DMFS) = DMFS2 - DMFS1, then it is easily shown that NI(DMFS) = DIF(DMFS). Many investigators define increments using change in the DFS scores for older adult populations because the reason for missing teeth is often unknown. However, the consequence of ignoring missing teeth in the definition produces inconsistencies between increment estimates because missing surfaces behave both like sound and "decayed" surfaces. If one were to estimate DFS increment by NI(DFS), the analog of NI(DMFS), where NI(DFS) = (SD+SF)-(DS+FS), it would no longer be equivalent to the analog DIF(DFS), where DIF(DFS) = DFS2 - DFS1 as one might expect. But rather the relationship between increment estimates now becomes DIF(DFS) = NI(DFS) + ("new lesions")-("reversals"). The "new lesions" (MD and MF) and "reversals" (DM and FM) terms are a consequence of not accounting for missing surfaces. Even if the true cause of tooth loss could be determined for all missing teeth, neither increment estimate appropriately counts surfaces for which caries progresses, treated or untreated, to the point where an extraction is performed.